The goal is to explore genetically-based methods for mapping activity in defined populations of neurons in the mouse olfactory system. The approach makes use of a genetically-encoded indicator for neuronal activity (synapto-pHluorin) in the intact mouse nervous system, and novel fluorescent proteins that can be spectrally separated from the indicator. The approach will be used to address fundamental questions of sensory processing by olfactory circuits. Specific Aim 1 is to test the physiological relevance of odorant receptor ligands by establishing an in vivo assay to image glomerular activity. Gene targeting will be used to label the glomeruli formed by olfactory sensory neurons that express defined odorant receptors. Responses from targeted glomeruli will be compared with responses from other glomeruli in the dorsal bulb to determine relative affinities for defined ligands. Previously identified odorants will be used and new chemical will be screened to identify high-affinity ligands in vivo. The experiments will provide a necessary context in which to place functional OR-ligand data from a variety of assays, and will pave the way for future experiments that aim to manipulate odorant receptor expression and olfactory function in mice. Specific Aim 2 is to target synaptopHluorin to mitral cells of the olfactory bulb to study lateral inhibition in the glomerular layer in vivo. This will permit, for the first time, selective imaging of synaptic release from mitral cell primary dendrites in vivo. The approach addresses a fundamentally important question regarding olfactory processing by olfactory bulb circuits that can not be addressed using currently available methods. These studies explore a new technique to set up experimental systems for studying neuronal function in vivo. Further development of these technologies promise to provide a novel window into brain function by allowing optical imaging of activity in genetically-defined neuronal populations that were previously difficult to isolate functionally in intact nervous tissue. [unreadable] [unreadable] [unreadable]